Meanwhile, federal prosecutors say they are expected to file formal criminal charges, possibly today, against Dzhokhar Tsarnaev that could lead to the death penalty, a decision being left to U.S. Attorney General Eric Holder.

Tsarnaev could possibly be slapped with a "weapon of mass destruction" charge, which carries a penalty of death. Authorities are using the public safety exemption that gives authorities the legal bases to question Tsarnaev without delivering Miranda rights. That's because authorities believe there is imminent threat out there.

Law enforcement sources say Dzhokhar Tsarnaev was awake and responding sporadically in writing to questions Sunday night. They are asking about any possible cell members and other unexploded bombs. There have been no additional details yet on his answers.

Tsarnaev is being treated at Beth Israel Deaconess Medical Center in Boston, where he is listed in serious but stable condition, with wounds to the neck and throat area, according to sources.

Last week's twin bombing killed three, including an 8-year-old boy, and wounded about 170. An MIT officer was allegedly killed by Tsarnaev and his older brother Tamerlan, and a Boston transit cop was badly wounded in a subsequent shootout.

New details emerged Sunday night about the dramatic capture after a 20-hour manhunt for Tsarnaev after he managed to escape a gunfight with police in Watertown, Mass., that killed his older brother.

As police and federal agents closed in after a Watertown resident reported there was a bloodied person hiding in his boat, a helicopter beamed back thermal images of the outline of Tsarnaev's body.

The orders went out to do everything to capture the teen alive even after an initial exchange of gunfire.

Police quickly surrounded the boat and there was a brief but ferocious volley of fire. Watertown Police Chief Edward Deveau told ABC News Sunday that negotiators demanded Tsarnaev show his hands and lift his shirt. Authorities were worried Tsarnaev was wearing a suicide vest. He was not.

"He was very slow and lethargic in every move that he made and they could see that there was no device on his chest. They kept creeping closer to him and then they felt it safe enough to pull him away from the boat," Deveau told ABC News' Martha Raddatz.

Nowhere were there more questions than at the University of Massachusetts Dartmouth, where Tsarniev was known as just an ordinary student.

"Literally, when I seen him he was just regular. He walked past me, we had a brief conversation and then I asked him for a ride home and he told me, 'Yeah,'" said Andrew Glasby, who lived at the same dormitory where Tsarnaev was staying after the marathon bombings.

Another friend, Zach Bettencourt, even asked Tsarnaev about the bombings.

"I talked to him in the gym about the bombing and he was like, 'Yeah man, tragedies happen all the time,'" Bettencourt said.

The night after the bombing, Tsarnaev tweeted, "I'm a stress free kind of guy."